1. Field of the Invention
This invention relates to compressed gas insulated transmission lines for high voltage electric power applications and particularly to longitudinal seams between arcuate sheath sections for forming a sealed cable.
2. Description of the Prior Art
In a typical compressed gas insulated transmission line (CGIT) system, a protective tubular sheath encloses and supports electrical conductors for power transmission. The sheath is filled with a gas, typically sulfur hexafluoride (SF.sub.6) pressurized at about 50 psig.
The major cost of a CGIT line system lies in the fabrication of the sheath. A large diameter conduit is typically constructed of extruded aluminum and consists of a plurality of modular elongated arcuate sections which readily fit together. This construction technique has the advantage that large diameter sheaths, i.e., greater than 20 inches in diameter, can be constructed at a lower cost than comparable spiral welded or rolled and welded pipe, as has generally been required in prior art high voltage isolated phase or three conductor CGIT line systems.
Segmented tongue-in-groove weld joints are known to the art. For example, U.S. Pat. No. 3,864,507, to Fox et al., describes a single conductor cable having longitudinal tongue-in-groove weld joints, wherein the interior of the cable is protected from the weld area. A construction, such as the Fox et al. design, has a number of disadvantages. First of all, the tongue-in-groove configuration requires very accurate extrusions of the tongue and mating groove. Extrusions of suitable accuracy are difficult and relatively expensive, particularly in long sheath segments. Furthermore, it is also very difficult to align the tongue-in-groove joint of large diameter, long length, multiple segment sheaths. Still further, an exterior weld along the joint may not be reliably uniform and may therefore have weaknesses which could develop into undesirable cracks if the seam is strained.
Therefore, an improved joint is needed wherein alignment is simplified, the joint is sufficiently strong, and construction time and costs are minimized.